1. Field of the Invention
The invention is generally related to nuclear reactor fuel assemblies and particularly to springs used in end fittings of the assemblies.
2. General Background
Typically, a nuclear reactor for the generation of electrical power includes a core of fissionable material to heat a coolant flowing up therethrough. The fissionable material is enclosed in elongated fuel rods assembled in a square array called fuel assemblies. The fuel rods are held in spaced parallel relationship by a number of spacer grids distributed at intervals along the length of the assembly. The fuel assemblies are held in an array by core grid plates at the top and bottom and are provided with upper and lower end fittings for mating with the grid plates. Typically, holddown spring means is provided between the upper end fitting and the upper core grid plate. This is necessary to provide sufficient holddown force against hydraulic lift forces in the core generated by coolant flow. The springs also allow for axial dimensional growth of the fuel assembly due to either differential thermal expansion or irradiation induced material change. The problems of design, therefore, are in the ability to provide sufficient holddown force against hydraulic lift while allowing sufficient room for growth. Sufficient material strength and stiffness must be available within a limited volume area. The stiffness/volume efficiency of a spring becomes very important when used for nuclear fuel holddown.
Known devices which address this problem include the following.
U.S. Pat. No. 4,551,300 entitled "Nuclear Reactor Fuel Assembly End Fitting" discloses an end fitting having a plurality of rigid levers and elastic means in recesses in the end fitting which exert a restoring torque on corresponding levels.
U.S. Pat. Nos. 4,072,562 and 4,072,564 disclose the use of torsion bars as holddown means.
U.S. Pat. Nos. 4,671,924; 3,801,453; 4,427,624; 4,631,166; and 4,420,457 disclose the use of leaf springs as holddown means.
U.S. Pat. No. 4,986,960 discloses the use of an end fitting having hairpin shaped springs along each side of the end fitting.
U.S. Pat. Nos. Re 31,583; 3,475,273; 3,515,638; 3,600,276; 3,689,358; 3,770,583; 4,076,586; 4,078,967; 4,192,716; 4,208,249; 4,278,501; 4,534,933; 4,560,532; and 4,729,868 disclose the use of a variety of holddown devices including helical springs and are representative of the general state of the art.
In certain reactor designs, the upper end fitting also has a pedestal attached to the grillage. The pedestal serves as a resting point for control rod and axial power shaping rod assemblies. It is from this location that the reactor control component drives as well as other handling systems (not in the reactor) connect with the control rod and axial power shaping rod assemblies. The force of the helical holddown spring used in these designs is transmitted to the reactor internals by way of a spring retainer. The holddown function of the spring and the interface function of the pedestal are independent. Helical springs present two problems. They are fully exposed to coolant flow which subjects the springs to the dynamic stresses of flow induced vibration. Also, reconstitution of a fuel assembly utilizing helical springs is a relatively complex operation. Known structures that use a pedestal affixed to the upper end fitting grillage do not permit the use of a central leaf spring and the advantages it affords relative to the helical holddown spring.